Nitride semiconductors having wide direct transition band gap, high breakdown electric field and high saturation electron velocity have been attracting attention as semiconductor materials for light-emitting devices such as LED and LD and high frequency/high power electronic devices such as HEMT (for example, see “Highly Reliable 250 W GaN High Electron Mobility Transistor Power Amplifier”, Toshihide Kikkawa, Jpn. J. Appl. Phys. 44 (2005), p. 4896 (Non-Patent Document 1)).
In order to reduce a cost of an epitaxial substrate and further to achieve a higher degree of integration with a silicon circuit device, research and development are conducted on the use of single-crystal silicon as a base substrate in manufacturing the above-mentioned nitride device (for example, see Japanese Patent Application Laid-Open No. 10-163528 (1998) (Patent Document 1), Japanese Patent Application Laid-Open No. 2004-349387 (Patent Document 2), and Japanese Patent Application Laid-Open No. 2005-350321 (Patent Document 3)).
Unfortunately, it is known that the formation of a good quality nitride film on a silicon substrate is much more difficult compared with the case of using a sapphire substrate or SiC substrate, from the following reasons.
First, silicon and nitride material are considerably different in the value of the lattice constant. This causes the generation of misfit dislocations at an interface between a silicon substrate and a deposited film or acceleration of a three-dimensional growth mode at the timing from nucleation to growth. In other words, this inhibits the formation of a good nitride epitaxial film that has a low dislocation density and has a flat surface.
Further, the nitride material has a larger thermal expansion coefficient value compared with silicon, and thus a tensile stress acts on the inside of a nitride film in the process of decreasing the temperature to about a room temperature after epitaxially-growing the nitride film on a silicon substrate at a high temperature. As a result, a crack is prone to occur on the film surface and a substrate tends to warp considerably.
In addition, it is known that trimethylgallium (TMG) being a source gas of a nitride material in vapor deposition is prone to form a liquid compound with silicon, which inhibits epitaxial growth.
From the reasons above, in the deposition of a nitride material on a silicon substrate, it is a significant technical challenge to, for example, inhibit dislocations or cracks in the film and further reduce warping of a wafer due to a stress, while maintaining the flatness of a film surface.
It is certainly possible to epitaxially-grow a GaN film on a silicon substrate in the cases of using the conventional technologies disclosed in Japanese Patent Application Laid-Open No. 10-163528 (1998), Japanese Patent Application Laid-Open No. 2004-349387, Japanese Patent Application Laid-Open No. 2005-350321 and Kikkawa. However, the GaN film obtained does not always have crystal quality that is as good as in the case of using SiC or sapphire as a base substrate. Thus, in the case of manufacturing, for example, an electronic device such as HEMT by the conventional technology, there arose problems of low electron mobility and low leakage current and low breakdown voltage.